The Slippery Facts of Oil-Change Intervals

Much of the marine industry is a more than a decade behind in its approach to oil-change intervals...

For as long as many of us can remember, the common wisdom on marine engine oil changes has been to drain and refill crankcase oil every 50 hours of operation.

A while back, recognizing that yachtsmen and commercial operators can blow through 50 hours in a week of running, some engine manufacturers officially loosened up their recommendations to allow for 100-hour intervals.

However, the current reality is that with a combination of regular periodic oil sampling and analysis, better oil filtering, and the seriously improved oils available today, significantly longer change intervals are possible. Cummins and Caterpillar, for example, both operate maintenance programs that support extending engine oil-change intervals effectively to 300 hours or more.

Well … It’s in the Oil

Engines have improved over the years. Overall, they run cleaner and foul their crankcase oil less. As well, oil filtration on contemporary engines is better than it used to be and, as a result, a greater percentage of particulates (the most serious category of oil contaminants) are picked up and removed from the oil. But the major factor in extending oil-change intervals is the improvements that have been realized in the newer synthetic motor oils.

Both conventional and synthetic motor oils are derived from crude oil. Crude oil is refined into petroleum oil — broken down into components that have different molecular weights, then separated into petroleum products such as gasoline, kerosene, and conventional motor oil. Synthetic oil is also refined from crude oil but is further manipulated by being “distilled” — broken down at a sub-molecular level and reassembled into purpose-specific molecules, kind of like GMOs, and finally, finished off with a custom-engineered blend of chemical additives.

The resulting “synthetic” motor oils exhibit 1) higher resistance to breakdown due to high temperatures, 2) less tendency to form sludge, 3) superior detergent properties, and 4) greater resistance to deterioration of lubricating properties. Their resistance to breaking down and/or losing some of their lubricating properties combines with their greater detergency (which holds particulate and other contaminants in suspension in higher percentages for longer) keep them working efficiently preserve engine parts and fight against engine wear.

The Key to Longer Oil-Change Intervals

Suggested oil-change intervals are only rough rules of thumb, constructed conservatively to cover the greatest number of individual cases at a minimum level of risk of being wrong for any given case. However, the actual rate of wear for any specific engine is related closely to the average operating conditions under which it is run. And that differs widely from engine to engine.

Nothing tells you more about how your engine is holding up against wear than tearing it down and examining its internal components. Unfortunately, tearing down your engine every month or so is not a practical solution, either in terms of time or dollars. But the next best thing is a program of regular periodic oil sampling and analysis.

Oil sampling and analysis are key in the process of matching oil-changes intervals to your engine’s actual operating conditions. However, it’s not just a matter of collecting a few samples and sending them to a lab.

Motor oil analysis screens for a multiplicity of indicators, including but not necessarily limited to abrasive particulates, sulfur (which can synthesize into corrosive sulfuric acid), glycol-based coolant, water, unburned fuel (all of which can degrade the lubricating properties of the oil), and soot (which can combine with other contaminants to form a gummy sludge that can block oil passages.

Because the precise levels of contaminants in normal operation will vary from engine to engine and with differences in operating conditions, the most significant metric is the rate of increase of these contaminants over regular intervals. Consequently, oil analysis is not a one-shot deal but really works only if a baseline is first established, then followed by regular periodic testing.

The Ideal Oil-Change Interval

If you followed me to this point, you’ll understand that there is no single “ideal” oil-change interval. Not even for a specified set of operating conditions (since different engines will perform differently in the same set of conditions). Nor even for a given make and model of engine (since the same make and model will perform differently in a different set of operating conditions).

Understand, as well, that not all engine “hour meters” are the same. Some measure what is known as “Hobbs time”, which is composed of hours and tenths of clock hours recorded by sensing via oil pressure or electrical activity when the engine is running. But some “hour meters” are tachometer rev counters (either mechanical or electronic). So not all “hours” are the same.

Even more important, 50 hours at running with a light load is very different from 50 hours running at a high load. The bottom line being nobody can tell you with certitude that you should change your engine oil every “x” hours of running time without normalizing load conditions and what they mean by “hour of running time”.

Which is why I’m not going to tell you what the ideal engine-oil change-interval is for your engine. What I will tell you is that very likely, if you use a high-quality synthetic motor oil of the correct specification for your engine, you will be able to run significantly longer than 50 engine hours between oil-drain changes.

Start with up-to-date recommendations from the manufacturer of your marine engine. Be aware that some owner’s manuals are often not updated on a timely basis, so you should check the latest data and recommendations in the manufacturer’s technical information pamphlets and brochures. And if you still find ambiguities, pick up the phone and call the engine manufacturer’s technical support or engineering department.

Often the answer will surprise you. Caterpillar, for instance, is now regularly allowing oil-change intervals of up to 250 hours, particularly if combined with participation is their S.O.S. oil monitoring program. Of course, they recommend you use their trademarked oil.

Keep in mind, as well, that there are limiting factors. For example, it’s probably not wise to lay up a yacht for the winter with used oil in its crankcase. A better practice is to change the oil at lay-up, run the engine for a short period to flush and dilute the used oil with fresh, and leave the engine laid-up with the fresh oil. If you choose to follow that practice, it’s ever so much more convenient, not to mention more economical, to be able to run the full operating season without an intermediate oil change. And that usually requires going more than 50 running hours between changes, so knowing the latest, up to date recommendation is well worth the effort.